252 The Data on which tve have to depend 



This difference for an area of 1,450 acres is three feet four 

 inches, and will suffice to supply 90,909, at forty gallons per 

 head, per day. 



The experiment on which Mr. Hodgkinson relies to prove 

 the evaporation from the surface of water, during three of 

 our summer months, has many singular features. 



It was conducted on a pond ou the banks of the Yarra, 

 very little above the sea level, and, therefore, in the most 

 favourable position to receive a lateral supply from higher 

 levels. Again, decomposed trap resting on stiff clay is ex- 

 ceedingly favourable to retain the winter rains from higher 

 levels, and to afford a large lateral supply to a pond fifteen 

 feet deep. 



It cannot be doubted that a large amount of water may be 

 supplied in this way. 



In many parts of Melbourne, and particularly at the lowest 

 levels, it is almost impossible to prevent the cellars being 

 filled with water. And, on the Grold-fields, the difficulties 

 that the dl2!:o;crs have to contend with from influx of water at 

 low levels, and in deep excavations, is well known. 



In selecting this pond for an experiment on evaporation, 

 especially when the justification of a vast expenditure of 

 public money depended on the result, it was incumbent on 

 Mr. Hodgkinson to show that It contained no springs, and 

 that there was no other indefinite source of supply that could 

 render the experiment fallacious. 



Springs are very often found in the ponds and water-holes 

 that form the beds of many of our creeks. This is a well 

 ascertained fact, and was therefore deserving of careful 

 consideration. 



In some instances the springs gush out of the rocks above 

 the water line, but, in general, they are principally distlngished 

 by the small apparent loss from evaporation in those ponds 

 in which they exist. 



The difference in this respect is very remarkable, where 

 there are chains of ponds all those without springs dry up 

 during the summer months, aud I have been assured by old 

 colonists, and residents on the Deep Creek, and other creeks, 

 that many of the ponds h^-ve from four to six feet of water in 

 them In November, and that they dry up completely in three 

 or four months. 



Nor can this be accounted for by any loss that might be 

 sustained from cattle drinking at them. Where there are 

 continuous chains of ponds it would be difficult to understand 

 how so many should be emptied in the same manner, and^ 



